The invention relates to a horn antenna for a radar device such as, for example, a radar-filling level measuring device. By means of a horn antenna of this kind, known as such for a long time, which is also designated xe2x80x9ccone antennaxe2x80x9d in the English-language literature, microwaves are radiated, which have been generated by HF energy coupled in. To be precise, short microwave pulses are radiated. In a combined transmitting and receiving system of a level measuring device equipped with such an antenna, the pulses reflected by the filling product are detected, and the distance from the filling product is assessed by measuring the transit time of these pulses. Radar-filling level measuring devices are, for example, used for a continuous level measurement of fluids, as well as of bulk goods or a combination of such products. Radar-filling level measuring devices are, for example, distributed under the trade name xe2x80x9cVEGAPULSxe2x80x9d by the company VEGA Grieshaber KG.
In radar devices having transmitting frequencies of under 8 GHz, basically two different antenna types are actually used. For antennas, which are not exposed to a heavy chemical load, metallic horns (preferably of stainless steel) are used. Thus, reference is made in an exemplary manner to the document DE 94 12 243 U1, which inter alia discloses horn antennas. For devices in highly aggressive surroundings or in applications in which the filling product to be measured for purity reasons is not allowed to get in contact with metal, metallic antennas of the mentioned kind, however, are not appropriate. For this purpose, it is proposed in the mentioned publication to provide a metallic horn antenna with a protective layer, which is corrosion-proof and permeable for microwaves. From the document WO90/13927, a horn antenna with a lens arrangement is moreover known, wherein the horn antenna, as well as the lens consists of a dielectric material such as, for example, polyethylene, polyester, cross-linked polysterol, glass fiber-enforced plastic or syntactic foam. A horn antenna with a filling of dielectric material is further known, for example, from JP 100 41 737, U.S. Pat. No. 4,161,731, and U.S. Pat. No. 4,783,665. By means of the filling consisting of a dielectric material, it is true, an improvement is achieved as compared to the aforementioned horn antennas; the production thereof, however, is still relatively complicated and expensive, and their mechanical stability still remains to be improved. Apart from that, complicated sealing mechanisms are necessary for their attachment.
Metallic horn antennas provided with a protective layer such as they had been initially discussed, cannot be used in the food sector due to their multipart mechanical structure, since here, a good cleanability is often required. In this case, so-called dielectric rod antennas (such as they are, for example, disclosed in the document DE 44 05 855 A1) are used. These are manufactured massive from chemically highly resistant PTFE (Teflon), but have various disadvantages as compared to the horn antennas. Thus, their function can be highly influenced by a condensate present on their surfaces. Due to the large active and mechanical length of such rod antennas, they are only conditionally suited for being built into a fitting of a vessel. Moreover, the performance cannot be enhanced in an arbitrary manner. Finally, the directional characteristic, as well, is less favorable than with a horn antenna.
The higher the selected transmitting frequency, the more important become the previously mentioned problems with rod antennas. Horn antennas, on the contrary, can be used without problems for higher frequencies, they just have to be adapted in their dimensions. With rod antennas namely, the rod thickness is a function of the frequency, it will more and more decrease with higher frequencies. Thus, the rod thickness of a rod antenna in the frequency range of 24 GHz would only be 4 mm (material PTFE), and the antenna would have a minimum length of 200 mm. Such antennas, however, can be easily damaged. Thus, a distortion would result in permanent damage of the antenna, or at least in a change in the antenna properties. Moreover, such antennas are much more sensitive to adherences or condensates, since in addition to the lower frequency, also a smaller surface is available. For materials suited for high frequency, e.g. ceramics, this construction cannot be realized. Due to the higher value, the diameter reduces again.
The technical problem on which the invention is based resides in providing a horn antenna for a radar device, which horn antenna can also be used in environmental conditions of a higher chemical aggressiveness, and at least reduces the hitherto arising disadvantages.
This technical problem is solved by a horn antenna comprising an antenna envelope defining the geometry characterizing a horn antenna. Hence, the antenna envelope delimits a flaring hollow space of the antenna, which space is at least in part filled up with a filling of a dielectric material, the opening of the hollow space of the hollow space of the antenna being closed by the filling towards the filling product present in a vessel. At its outside, the antenna envelope is likewise enveloped by a dielectric material and namely so that the filling of the antenna""s hollow space and the coating on the outside in toto screen the antenna envelope against a possibly chemically aggressive atmosphere in a vessel filled with the filling product to be measured.
Preferably, the antenna envelope as a whole, i.e. on its inner and outer side, is embedded in the dielectric material so that the outer encasing of dielectric material in combination with the filling has a stability-increasing effect. The outer encasing, in particular, constitutes preferably a cylindrical base body, or a conical base body tapering towards the filling product.
In a preferred embodiment, the outer encasing encloses also the opening of the hollow space, i.e. it is configured as a unilaterally open pot. A further development of this embodiment provides the radiation surface of the envelope being then configured as a (concave or convex) lens, resulting in the improvement of the radiation characteristic.
In a preferred embodiment, it is provided that the antenna envelope consists of metal, which envelope is then enveloped inside and outside by the dielectric material, and is in particular completely embedded therein. Alternatively, it is also possible to produce the antenna envelope itself from a dielectric material, which antenna envelope then being embedded in another dielectric material. The respective materials then have to be selected with respect to the radiation characteristic.
The invention is based on the idea of filling up, at least in part, the inner space (hollow space) enclosed by the horn antenna with a filling material for stabilizing and increasing the mechanical stability, and of preventing therewith at the same time condensate or such like from penetrating into the hollow space, and moreover, of applying a dielectric filling material also on the outside of the antenna envelope. Through this combination of a dielectric encasing on the inside and the outside, the envelope is protected and of a higher mechanical stability.
Preferably, at least those surfaces of a usual metallic horn antenna are provided with a protective coating, which could come into contact with the chemically aggressive atmosphere in a vessel filled with the filling product to be measured. Therewith, it is achieved that, for one, a horn antenna can now also be used in environmental conditions which hitherto required other types of antennas, and, for another, the pressure support is improved in such atmospheres.
An inventive filling can therewith also act at the same time as a protective layer. In case the filling product has only a mechanically supporting quality, the application of a separate protective layer to those metallic parts or surfaces of the horn antenna is possible, which come in contact with the atmosphere of higher chemical aggressiveness. By filling up the inner antenna space with a filling product, the pressure support is at the same time considerably increased.
The most diverse materials can be used for the filling. From the production-technical point of view, it is extremely advantageous to use, for example, a foam (such as, e.g. ROHACELL) or another material, the e, of which is close to air. In certain applications, however, it is also possible to provide not only the coating but also the filling made of a dielectric material, such as, e.g., PTFE. For a high mechanical and thermal stability, a fiber-enforced synthetic material is recommended such as, e.g., PPS or PP. Such fiber-enforced synthetic materials can also be used in combination with other suitable materials. Thus, the coating can be applied by stoving, sintering or such like. In case the filling is made of a dielectric material, the filling, as well, can of course be manufactured likewise.
When used in explosion-endangered areas, it can be advantageous to coat the dielectric surface of the antenna with a conductive layer such as it is, for example, known from the document DE 196 17 963 A1. By means of such a layer, the electrostatic charging of the horn antenna is prevented.
For simplifying the sealing of such a horn antenna, it is particularly purposeful to apply the envelope of dielectric material also to a flange for the attachment of the horn antenna, namely, on the side directed into the radiation direction of the horn antenna. Thereby, a sealing between the flange and the vessel is achieved in an optimum manner, and the flange is at the same time protected from the chemically aggressive atmosphere within the vessel. Moreover, there is no need to add any separate sealing parts.
Preferably, such a horn antenna for a radar-filling level measuring device hence features an antenna envelope having the geometry characterizing a horn antenna, and being filled up inside, at least in part, with a filling of dielectric material, which closes the opening of the hollow space of the hollow space of the antenna, and which is moreover at the same extended to the side of an attachment flange of the antenna directed into the radiation direction. Preferably, such an antenna is also enveloped outside with the dielectric material, such as it has been initially discussed above.
A preferred embodiment comprises an antenna envelope consisting of metal and having the form of a cylinder, which at its inside is provided with a cone-shaped hollow space passing over into a cylindrical hollow space. Into this cylindrical hollow space extends an exciter pin, which radiates a microwave signal generated in an electronic unit into this hollow space. In the hollow space of the antenna itself, a filling formed from a dielectric material in the form of a cone is present, the tip of which extends into the cylinder-shaped hollow space in the antenna envelope. The cone tip preferably comprises a slightly larger angle inclination than the rest of the filling. Hereby, the radiation characteristic is improved. The angle difference preferably is 2-3 degrees.
According to another aspect of the invention, the inside filling consists of several parts of different dielectric materials. The outside encasing is formed by a part of dielectric material connected with the inside encasing, which part radially extending so far to the outside that it screens a flange situated behind same, which serves for attaching a filling level measuring device from the atmosphere within the vessel, when it is in the attached condition.
It is particularly advantageous to screw the different parts of the filling and the outer encasing with each other, and to bond one or more parts of the inner filling with the antenna envelope by adhesion. Thereby, an optimum radiation is made possible, and at the same time even a high temperature resistance and adaptation to various temperature expansion coefficients is achieved. As the material, in particular PTFE, epoxy resin-bound hollow glass microballs and modified Teflon is used.
It has still to be noted here that of course all of the features and technical details discussed in the present description as to a specific embodiment of the invention can be arbitrarily combined with each other, and can also be used in another embodiment of the invention without problems, taken alone or combined with each other. This will depend on the respective conditions of use and the filling level measuring device.